A federal judge on Wednesday blocked enforcement of a provision of California's newly approved sex-trafficking law that requires 73,000 registered sex offenders to reveal their Internet identities to police.

A day after voters overwhelmingly approved Proposition 35, U.S. District Judge Thelton Henderson of San Francisco said civil liberties groups had raised "serious questions" about whether the online disclosure requirement violates freedom of speech.

Quoting Attorney General Kamala Harris' office as saying the state would not be able to enforce the disclosure requirement until March, Henderson issued a temporary restraining order that prohibits enforcement at least until Nov. 20, when he will consider a request for a longer-lasting injunction.

Prop. 35, approved Tuesday by 81 percent of the voters, increases prison sentences for trafficking offenses, such as coercing someone into prostitution. It also requires anyone registered as a sex offender, for crimes ranging from indecent exposure to rape, to provide police with their e-mail addresses, Internet user names and the names of their Internet service providers.

In a lawsuit filed Wednesday, the American Civil Liberties Union and the Electronic Frontier Foundation argued that the disclosure requirement violates free speech because it interferes with the right of an individual to anonymously participate in online discussion forums and puts paroled offenders at risk of exposure and retaliation if they use the Internet.

"The ability to speak freely and even anonymously is crucial for free speech to remain free for all of us," said ACLU attorney Michael Risher. "Stopping human trafficking is a worthy goal, but this portion of Prop. 35 won't get us there."

But Prop. 35's supporters said the online disclosure requirement was modeled on provisions that have survived legal challenges in other states, including a Utah law upheld by a federal appeals court in 2010.

Police will use the Internet identities "to assure that convicted sex offenders are not using those (websites) to commit new crimes," said Chris Kelly, a sponsor of Prop. 35 and similar laws in other states and former chief privacy officer of Facebook. He said Prop. 35's sponsors would seek to join the state in defense of the initiative.

The lawsuit was filed on behalf of two unidentified former sex offenders and an advocacy group called California Reform Sex Offender Laws. The suit said the men post anonymous comments about those laws and other issues on the group's website but would be deterred from doing so if forced to reveal their identities to police.

One plaintiff has already left the state because of the prospect of having to disclose his name, the suit said. It said the other plaintiff, a 75-year-old Alameda man who has been crime-free since his release from custody in 1991, "fears retaliation and social stigma if he and other registrants are unable to participate anonymously in online dialogue."

State records show that registered sex offenders are rarely accused of using the Internet to victimize children, the suit said, and disclosure should be required only for those shown to pose a high risk of such offenses.

Similar arguments were made against an identical disclosure requirement in Utah. The federal appeals court in Denver found no constitutional violation, saying the law required police to keep the information confidential and use it only to investigate a new crime.